1. Field of the Invention
This invention relates to an improved method for synthesis of diamond and an apparatus used therefor. More particularly, it relates to a method for synthesis of diamond by a microwave plasma CVD method according to which area of synthesis of diamond can be markedly enlarged as compared with conventional methods and an apparatus necessary for practice of the method.
2. Description of the Related Art
Recently, technique for synthesis of diamond has been remarkably developed and synthetic diamond has been widely used, for example, as various protecting films, optical materials, electronic materials and chemical industry materials.
For forming a thin film of diamond on the surface of a substrate, it is known to form a diamond thin film on the surface of substrate by deposition of diamond by various means such as chemical vapor phase deposition method (CVD method), ion beam method, and chemical transport method using raw material gas containing a carbon source.
Especially, method of forming a diamond thin film at low temperatures by the chemical vapor phase deposition method has been recently noticed because continuous operation is easy and this method is industrially advantageous.
It is important for industriallization of this CVD method how to activate a mixed gas. There have been known (1) the hot filaments CVD method by which a mixed gas is led to activated state by passing it near ignited filaments, (2) the high frequency plasma CVD method by which a mixed gas is led to activated state by applying high frequency to introduction part of a mixed gas to produce plasma there by the high frequency, (3) the microwave plasma CVD method by which a mixed gas is led to activated state by introducing microwave into gas introduction part to form plasma there by the microwave.
Among them, especially the microwave plasma CVD method is superior in reproducibility of activated state and gives no consumption parts and this is promising.
However, since in the conventional microwave plasma CVD method, microwave oscillated from one microwave oscillator is introduced into plasma generator [Japanese Patent Application Kokai (Laid-open) No. 91100/83], area of synthesis of diamond depends on wavelength of microwave and when a microwave of 2.45 GHZ is used, about 2 inches in diameter is maximum. This, there are problems that the area of synthesis is small and use of diamond produced is limited.